I. Field of the Invention
This invention concerns a method of fabricating permselective membranes suitable for the separation of components of fluids. In one embodiment, the fabricated membrane may be in the form of a hollow fiber with a barrier layer on its external surface having the capacity for separation under applied pressure the components of a gaseous mixture such as air. In other embodiments the invention may be in the form of a hollow fiber having the capacity for separation under pressure the components of a liquid. In other embodiments, the invention may be in the form of tubes, flat sheets, spiral-wound, or pleated sheets.
II. Brief Description of the Prior Art
There is a very substantial technical and patent literature extant describing the presently known wide range of synthetic membrane types and methods of producing them. Extensive contemporary treatment of the field will be found in:
Kesting, R. E. - "Synthetic Polymeric Membranes Structural Perspective" - 2nd Ed., Wiley, N. Y. 1985. PA1 Belfort, G. - "Synthetic Membrane Processes", Academic Press, Inc., Orlando, Fla., 1984. PA1 Sourirajan, S., and Matsuura, T., - "Reverse Osmosis and Ultrafiltration" - ACS Symposium Series #281, American Chemical Society, Washington, D.C., 1985. PA1 Lloyd, D. - "Materials Science of Synthetic Membranes", ACS Symposium Series #269, American Chemical Society, Washington, D.C., 1985. PA1 Turbak, A. - "Synthetic Membranes" - 2 Vols., ACS Symposium Series #153, American Chemical Society, Washington, D.C., 1981. PA1 Cooper, A. R. (Ed.) "Ultrafiltration Membranes and Applications" Plenum Press, N.Y., 1979 (Chapter by Cabasso) PA1 Cadotte, J. E. "Interfacial Thin-Film Composite Membranes" Symposium sponsored by Bend Research, Bend, Oreg. 1983. PA1 Schwartz, H., et al - "Skin Layer Characterization of Anisotropic Membranes for Ultrafiltration", J. of Membrane Science, Elsevier Amsterdam, 1982. PA1 Wrasidlo, W. & Mysels, J. - "The Structure and Some Properties of Graded Highly Asymmetrical Porous Membranes" - J. of Parenteral Science and Technology, Vol. 38, Jan.-Feb. 1986. PA1 1. the parent polymer of the asymmetric membrane has desirable selectivity; PA1 2. an ultra-thin dense barrier layer free of imperfections or pores in an asymmetric structure cannot be achieved by the measures ordinarily practiced; PA1 3. the pores in the barrier layer must be plugged with another material; PA1 4. this is accomplished by applying a plugging material (generally a polymer in solution) to the external face of the barrier layer under the action of applied positive pressure to drive some of the plugging material into the pores; PA1 5. the method inevitably also leaves a layer of the plugging material deposited on the non-porous areas of the barrier; PA1 6. in consequence, it is necessary to employ a plugging material with intrinsic high permeability; PA1 7. and materials of that quality inevitably have low intrinsic selectivity.
Other material specifically relevant to the subject matter of this invention will be found in:
It is well known that three major factors determine functional effectiveness of a practical membrane: the chemical nature of the membrane polymer; the fine physical morphology of the membrane polymolecular structure; and gross configuration. Gross configuration is meant to note such geometric distinctions as are characterized by the generic terms--"flat sheet", "spiral wound", "tubular", "hollow fiber", etc. Fine physical morphology is meant to denote the qualitative and frequently quantitatively definable features of membrane architecture at the level of the dimensions of aggregates of a few dozen polymer molecules. Chemical factors of a membrane polymer is meant to denote the nature of the atoms comprising the polymer and the system of primary and secondary valence bonds by which they are linked.
It is generally agreed that both physical morphology and polymer chemistry play important roles in the mechanisms by which ions or single uncharged molecules of a fluid mixture are selectively sorbed and subsequently migrate through a membrane. It is also generally understood that the selectivity of a membrane, whether for gases or for molecules and ions in solution, or even for larger species such as colloidal particles, is a function of the membrane upstream surface to the depth of only a few molecular radii. Permeability, however, is a function of not only the membrane chemistry but the thickness of the selective layer as well as hydraulic resistance factors determined by the morphology of the entire membrane cross-section.
By way of illustrating the importance of polymer type on selectivity, the following table presents the values of permeability of oxygen, nitrogen and helium as reported by several authorities and normalized here for comparative purposes to what could be expected from a test on a perfect dense film 1.mu. thick.
TABLE I ______________________________________ Permeability and Selectivity of O.sub.2, N.sub.2 and He in Several Polymers P/t*.sup.(b) .alpha. Polymer.sup.(a) Reference O.sub.2 N.sub.2 He O.sub.2 /N.sub.2 He/N.sub.2 ______________________________________ CA Kammermeyer .018 .003 -- 5.9 CT Toyobo .015 .0025 -- 5.9 EC Kammermeyer .12 .03 -- 4.0 Si Fr.1,379,288 7.0 3.1 -- 2.2 PS Erb & Paul .017 .003 -- 5.8 CA Gantzel & -- .003 .29 -- 95 Merten PS US 4,230,463 .018 .003 .09 6.0 30 PS UCC .017 .003 -- 5.8 50 PMMA Chiou (U.Tex) .0013 .00016 -- 8.0 ______________________________________ .sup.(a) CA is cellulose 2.7 acetate; CT is cellulose triacetate; EC is ethyl cellulose; Si is silicone rubber; PS is bisA polysulfone (Udel3500) PMMA is polymethylmethacrylate .sup.(b) Expressed in units of cubic feet/sq.ft./day/psi.DELTA.P for a dense film with thickness, t = 1.mu. (10,000.ANG.).
A review of the variable contributions of chemistry and morphology will be found in C. E. Reid and E. J. Breton (J. of App. Poly Sci., 1959, pp. 133-143). A more extensive discussion will be found in a chapter by P. Blais in a book edited by Sourirajan (Reverse Osmosis and Synthetic Membranes, 1977, National Research Council of Canada, Publication #15627, Ottawa). It is useful to consider some of the variations of morphology and to some extent the chemistry of existing membranes in order to establish the novelty of the present invention.